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I'm having some trouble comprehending the following image. I am supposed to be able to deduce from the figure that the total difference in phase angle is (k-k')r. To begin with, I do not understand why for the diffracted wave the difference in phase angle is -k'*r while for the incomming wave it is just k*r. Where does the minus sign come from?

Thanks in advance...

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# Total phase difference diffraction help

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